1. Field of the Invention
The present invention relates to a camera which is capable of accommodating interchangeable lenses and which has focus detection capability. More specifically, the present invention relates to a camera which decides whether or not focus detection is possible based upon characteristic data for an interchangeable lens mounted on the camera and upon characteristic data for an automatic focus device located in the camera.
2. Description of the Related Art
Cameras which can accommodate interchangeable lenses and which have focus detection capability are known. For example, Japanese Laid-Open Patent Publication No. 63-284526 ("JP-A-63-284526") discloses a camera wherein data representing an exit pupil of an interchangeable lens (i.e., the stop aperture of the interchangeable lens) is stored in the interchangeable lens and is used to decide whether or not focus detection is possible. However, the exit beam through the interchangeable lens happens to be limited by the placement of the edge of a lens block (e.g., the lens block may be placed in front of or behind the stop aperture). Therefore, the exit beam generally cannot be determined at the exit pupil (stop aperture).
The above-described phenomenon is referred to as "vignetting", and the phenomenon becomes evident when the image height increases. It is well known to limit the vignetting phenomenon of the exit beam by using an aperture outside the stop aperture, with the result that when image height increases, the amount of exit light becomes small. In known cameras, information concerning the exit beam was considered necessary for the cameras with focusing capability. However, as described above, if the exit pupil stop aperture does not completely determine the exit beam, the information relating to the exit pupil is insufficient to determine whether or not focus detection is possible.
According to JP-A-63-284526, an image height correction quantity is stored. However, the image height correction quantity is for the situation in which there is no vignetting. Because the exit pupil is seen obliquely when the image height increases, the exit pupil becomes flattened, and the amount of its correction does not correctly represent the exit beam.